1. Field of the Invention
The present invention relates to an apparatus and a method for assessing the lifetime of an oil-filled electrical device, and to an apparatus and a method for suppressing degradation of an oil-filled electrical device.
2. Description of the Background Art
An oil-filled electrical device, particularly oil-filled transformer has a coil wire and insulating paper wrapped around the wire. The insulating paper provides electrical insulation between adjacent coil turns. While the transformer is used for a long period of time (several decades for example), the average degree of polymerization of cellulose molecules that form the insulating paper gradually decreases. Accordingly, the mechanical strength of the insulating paper gradually decreases.
In the case where short-circuit current flows in the transformer due to a system trouble, an electromagnetic force acts on the coil. The electromagnetic force is determined depending on the short-circuit current. When a large short-circuit current flows, a large electromagnetic force is generated on the coil and thus a tensile force is exerted on the coil insulating paper. When an excessive tensile force acts on the insulating paper which has been deteriorated, the insulating paper is broken. The breakage of the coil insulating paper causes deterioration of the electrical insulation property between adjacent coil turns. This is a typical mechanism that has a predominant influence on the lifetime of the transformer. It is therefore indispensable for assessment of the lifetime of the oil-filled electrical device to estimate the mechanical strength of the coil insulating paper.
As a method for preventing short circuit between coil turns due to decrease in mechanical strength of the insulating paper, an electrical device lifetime assessment method based on the degree of polymerization of the insulating paper has been proposed. The degree of polymerization of the insulating paper has a correlation with the mechanical strength of the insulating paper. The degree of polymerization of the insulating paper is therefore used for assessment of the lifetime of the electrical device (Patent Document 1: Japanese Patent No. 3516962 (WO98/056017)).
Patent Document 1 discloses a mathematical expression for calculating the degree of polymerization of insulating paper from the heating temperature and the heating years. According to Patent Document 1, the phenomenon of thermal degradation of insulating paper is different depending on whether the temperature is above or below 110° C. The above-referenced mathematical expression is derived from experiments in which an insulating oil having insulating paper therein is heated at a temperature of not more than 110° C. for a maximum of 12 years.
The relation between the mechanical strength of insulating paper and the degree of polymerization of the insulating paper is determined in advance. The degree of polymerization at the time when the mechanical strength of the insulating paper reaches a design limit value is the design limit value of the degree of polymerization of the insulating paper. The lifetime of the oil-filled electrical device can be assessed by estimating the degree of polymerization of the insulating paper.
A problem that a copper sulfide causes dielectric breakdown in an oil-filled electrical device has recently been reported. A sulfur component contained in an insulating oil reacts with a copper component in the insulating oil, so that an electrically conductive copper sulfide is deposited on insulating paper. The copper sulfide deteriorates the insulating property of the insulating paper. The deteriorated insulating property of the insulating paper results in dielectric breakdown (Non-Patent Document 1: CIGRE WG A2-32, “Copper sulphide in transformer insulation”, Final Report Brochure 378, 2009).
Conventional lifetime assessment methods do not consider degradation of the insulating property due to deposition of the copper sulfide on the insulating paper, Therefore, the conventional methods may not correctly assess the lifetime of the oil-filled electrical device in some cases. In order to accurately analyze the state of the oil-filled electrical device, it is required to appropriately identify a main determinant that determines the lifetime of the oil-filled electrical device.